Former Sociedad president Inaki Badiola revealed earlier this month that the Basque outfit bought banned substances during a six-year period between 2001 and 2007, a claim that has since been denied by the club, and Alonso has now stressed that he was unaware of any alleged doping use.

"During my time playing for Real Sociedad, I never saw or knew of anyone who was involved in using these kind of substances," the midfielder was quoted as saying by The Times.

"Real Sociedad has always been an impeccable club and it will continue the same way."

The 31-year-old then went on to discuss the upcoming Champions League round of 16 tie between Real Madrid and Manchester United, and stated that Cristiano Ronaldo could prove to be the difference between both sides.

"Probably the thing that people don’t realise is how professional Ronaldo is. He looks after himself so well, trains hard every day and wants to be the best that he can.

"That is how competitive he is. Maybe he gets some of that from [Sir Alex] Ferguson also, but wherever it comes from, the statistics show that he is up there with [Lionel] Messi as [one of] the best players in the world.

"He will love going back to Old Trafford. You only have to think of how much he was adored there to realise the kind of reception that will be waiting for him. As soon as we knew what the draw was, he was really excited about it."

The first leg takes place at the Santiago Bernabeu on Wednesday, with the return scheduled for March 6 at Old Trafford.